1. Field of the Invention
The invention relates to a process for the reversible optical storage of data using polymeric liquid crystals.
2. Discussion of the Background
Between the solid crystalline phase and the fluid melt, designated hereafter as an isotropic melt, intermediate phases appear in certain substances, which from a structural and dynamic viewpoint combine properties of both the ordered, crystalline state and the disordered melt state within themselves. Thus such phases are fluid, but have, for example, optical properties which are characteristic of most of the crystalline and partially crystalline substances, i.e., they are birefringent. They are called for obvious reasons intermediate phases (mesophases) or liquid crystal phases. These intermediate phases may be obtained by a temperature variation (thermotropic liquid crystals) or in solution by means of concentration variations. Only thermotropic liquid crystals shall be considered hereinafter. To characterize the range of existence of these intermediate phases, the transition temperatures from the crystalline state into the liquid crystal state and from the liquid crystal state in the isotropic state clearing temperature, determined, for example, by calorimetry or by a polarizing microscope, are generally given. In case of the presence of different liquid crystal states, a set of corresponding transition temperatures is given.
The appearance of mesphases is associated with peculiarities in their molecular geometry. Spherically symmetric molecules cannot form mesophases. In contrast, molecules which may be characterized as cylindrical or disc-like can form mesophases. The molecules may be rigid and the ratio of their maximum to thrir minimum dimension (for example, length of cylinder/diameter of cylinder) must clearly exceed a critical value of about 3.
The structure of such mesophases is characterized in that in the simplest case for cylindrical molecules, in the so-called nematic phase, the molecular centers are distributed in a disordered manner as in an isotropic melt, while the longitudinal axes of the molecule are parallel to each other. This differs from the state in the isotropic melt, in which the molecular axes are statistically distributed. This results in anisotropic mechanical, electrical or optical properties. In the cholesteric phase, an additional ordering principle is present, i.e., a continously helical variation of the direction of orientation of the longitudinal molecular axes appears, leading to particular optical properties, such as strong optical activity or the selective reflection of light. Finally, in the so-called smectic phases there occurs in addition to the aforedescribed orientation order characteristic of the nematic state, a uniform arrangement of molecular centers of gravity in space, for example, along one spatial axis only, or in other smetic modifications along two or even three independent axes. In spite of this, these phases are fluid.
Disc shaped molecules are capable of forming so-called discotic phases, wherein either only the disc normals are oriented parallel to each other (see the nematic phase) or in which the discs are arranged within columns in a regular or irregular manner. These are referred to as columnar structures.
A characteristic value which is highly important for the application of liquid crystal structures is the orientation order parameter, which is a measure of the quality of the orientation order.
Its value is between 0, in case of complete disorientation (as in the isotropic melt), and 1, when all of the molecular longitudinal axes are oriented in a perfectly parallel manner.
The widespread application of liquid crystal substances in industrial products, such as display elements in pocket calculators, wrist watches or digital measuring instruments, is based on the particular property that the direction of orientation, which may be represented by the so-called director, is readily varied by external electrical, magnetic or mechanical fields. The resulting changes in optical properties may be used in combination with other components, such as polarizers, cell walls, etc., in display elements for the visualization of information. The cell walls serve to protect the fluid mesophases and determine the macroscopic configuration of the liquid crystal film required.
It has been discovered in recent years that in numerous fields of application it may be advantageous to combine the properties of liquid crystal phases with those of polymers. The advantageous polymer properties are good mechanical properties, making possible the production of thin, dimensionally stable films of these substances, together with the occurrence of a freezing process (glass transition), whereby the fixation of a predetermined orientation structure is made possible. The citation of the glass temperature (T.sub.g), determined for example, by calorimetry, serves to characterize the range of existence of the solid liquid crystal phase. Above this temperature the polymer is in a viscoelastic or viscous plastic state.
Theories of the formation of liquid crystal phases generally and the formation of such phases in polymer systems in particular, together with experimental results, show that liquid crystal polymers can be produced from rigid mesogenic structural units which are characteristic of low molecular weight liquid crystals, in combination with flexible spacer groups and flexible chain molecules. In this process, different structural features are possible. The mesogenic groups are in the case of the class of side chain liquid crystals attached to a flexible or semiflexible main chain, by means of a spacer or even without one. The mesogenic groups may be cylindrical or disc-shaped. The main chain may contain mesogenic groups separated by flexible units. Copolymers, characterized in that different spacer and/or mesogenic groups appear within a single polymer, are also capable of forming liquid crystal phases.
In addition to these side chain liquid crystals, main chain polymers also exhibit liquid crystal phases under certain conditions. The conditions for this are that either the chains consist entirely of rigid groups, or of rigid and flexible groups. Copolymers of different mesogenic groups and/or spacer groups can also form liquid crystal phases. Mesogenic groups are of a cylindrical or rod-shaped configuration. The nature of the mesophases, the range of their existence and that of the glass phase may be adjusted approximately by means of the spacer length and flexibility, the flexibility of the main chain and its tacticity and length.
Heretofore only main chain polymers, exclusively with rigid units or overwhelmingly with rigid units, have been introduced practically in the market. They have extremely high strength and rigidity values. These self-strengthening thermoplastic synthetics are known. Their field of application consists of mechanical parts requiring extreme mechanical properties (Kirk-Othmer, Encyclopedia of Chemical Technology, 3rd ed., Vol. 14, pp. 414-421 (1984); J. H. Wendorff, Kunststoffe 73, 524-528 (1983); M. G. Dobb, J. E. McIntyre, Adv. Polym. Sci. 60/61, 61-98 (1984).
Polymers with flexible and rigid units have not been used heretofore in systems introduced in the market. Their advantage consists of having a high orientation parameter value compared to side chain liquid crystals (C. Noel, F. Laupretre, C. Friedrich, B. Fagolle, L. Bosio, Polymer 25, 808-814 (1984); B. Wunderlich, I. Grebowicz, Adv. Polymer Sci. 60/61, 1-60 (1984). Polymers with mesogenic side chains have also attracted attention in recent times (H. Finkelmann in "Polymer Liquid Crystals", Academic Press (1982); H. Finkelmann, G. Rehage, Adv. Polym. Science 60/61, 99-172 (1984); V. P. Shibaev, N. A. Plate, Adv. Polym. Science 60/61, 173-252 (1984).
From US 4,293,435 an industrial utilization of the specific behavior of liquid crystal polymers, connected with the transition into the glassy state, is known. Information is stored by the application of conditions which alter the arrangement and orientation of liquid crystal polymers in a defined manner (for example, electrical or magnetic fields). This is also discussed in GB No. 2,146,787. It is pointed out that the storge of the device provided for in U.S. Pat. No. 4,293,435 in the solid state below the glass temperature (T.sub.g) signifies that T.sub.g is above the usual room temperature (T.sub.a), i.e., that the polymer system is used at temperatures which are higher by about 100.degree. C. over T.sub.a, if the information is to be stored within a reasonable period of time. Such temperatures are unwieldy and lead in the long term to a decomposition of the polymer. According to GB No. 2,146,782, these difficulties may be avoided by the use of certain polymeric side chain liquid crystals. It is then no longer necessary to store the device at a temperature range below T.sub.g and stable storage for many years is possible at temperatures above T.sub.g and below a temperature (T.sub.f), at which the polymer begins to liquify.
The T.sub.f may be determined by ovserving the passage of light through a liquid crystal polymer between two crossed polarizing filters at increasing temperatures starting from the glass temperature. A few degrees below the smectic-isotropic phase transition the transmission of light suddenly increases. This rise is the result of the transition of an anisotropic low transparency state to a highly birefringent, transparent state of the zone. The temperature range above this temperature T.sub.f is designated the "fluid region". The transparency increases with rising tempeatures until it attains a maximum at T.sub.m. T.sub.m marks the point at which the isotropic (clear) phase first appears.
Since the appearance of the isotropic phase with crossed polarizers leads to the extinction of light, a further increase in temperature results in a decrease in the passage of light as the isotropic regions increase, until the so-called clearing temperature (T.sub.c) is attained, at which the last remnants of the structure responsible for birefringency have disappeared.
GB No. 2,146,787 claims an apparatus with a material layer containing a liquid crystal polymer with a mesogenic side chain, together with devices for the thermal conversion of at least part of the material from the viscous state, in which the temperature of the material is within the range of T.sub.g to T.sub.f, to the liquid state. Also claimed are devices to affect at least part of the material in the liquid state, whereby a selective change in the texture of the molecule in the material is effected and information stored, which is retained even after the cooling of the liquid region and the return to the viscous state. It is therefore an essential condition of GB 2,146,787 to use a polymer material for which T.sub.f &gt;T.sub.a &gt;T.sub.g. A device is further described, in which the material layer contains a liquid crystal polymer with a smectogenic side chain. Particularly preferred are polymeric liquid crystals of the polysiloxane type with diphenylcyanogen side chains or benzoric acid ester side chains.
Great interest still exists in optical storage media which in addition to high recording densities are also capable of reversible storage. The above-described solutions of the problem of optical data storage represent relatively narrow technical solutions. Thus, the device according to GB No. 2,146,787 is based on the use of liquid crystal side chain polymers with the essential condition that the temperature be selected in a manner such that the polymeric material is maintained in a viscous state. The disclosure extends to polysiloxane liquid crystals, preferably with diphenylcyano or benzoic acid ester side chains. The stability of the information stored is not unambiguously guaranteed in view of the existing molecular mobility and the finite relaxation times and also of possible effects on the system, for example, by external interfering fields. There remains a need for technical solutions whose limits are not too narrow.